The present invention relates to a control circuit for a workpiece handling device and will be disclosed in connection with an apparatus for aligning and treating flattened square pieces of thermoplastic resin for placement upon a conveyor. More specifically, the invention concerns a workpiece detection circuit which is used in the preferred embodiment to monitor a location of the apparatus and to generate an error signal whenever an unanticipated workpiece presence or absence occurs during selected sampling periods.
Almost all article handling apparatuses are subject to occasional, albeit infrequent, misoperations. In many instances these misoperations result in "jamming" of the workpieces being handled. Once "jamming" has occurred, continued workpiece feed to the apparatus may operate to the detriment of the apparatus. Accordingly, it is desirable in such circumstances to terminate workpiece feed and to reduce the possibility of compounding and aggravating resultant problems.
The present invention is directed toward alleviating the above-mentioned problems. It samples a selective location of the apparatus during a first sampling period when a workpiece is anticipated at that location and generates an error signal if this expected condition is not met. The same sensor tests for the absence of a workpiece during a second sampling period in which no workpiece is anticipated and generates an error signal if a workpiece is sensed.